Nonvolatile floating gate transistor structure

Abstract

A field effect transistor includes a source region, a drain region, and a channel region formed in a semiconductor substrate and a floating gate and a control gate formed over the substrate. An opaque cover (typically aluminum) is formed over but electrically insulated from the transistor to prevent light from striking and affecting the electrical charge on the floating gate. The periphery of the opaque cover ohmically contacts the semiconductor substrate, thereby limiting the amount of light reaching the floating gate, except where the source and drain extend inwardly beyond the periphery of the opaque cover. The control gate extends over a portion of the substrate surrounding the transistor, and helps hinder light from reaching the floating gate. In addition, semiconductor material formed concurrently with the control gate extends over the source and drain regions, thereby providing additional shading.

We claim: 
     
       1. A transistor comprising: a region of semiconductor material;   a source formed within said semiconductor material;   a drain formed within said semiconductor material;   a floating gate formed above at least a portion of the part of said semiconductor material between said source and drain; and   means for preventing light from reaching said floating gate, said means for preventing comprising: a control gate, said control gate extending over said floating gate, said source, said drain, and the portion of said semiconductor material surrounding said floating gate, said control gate being opaque, said control gate hindering light from reaching said floating gate and removing electrical charge stored on said floating gate; and   an opaque cover formed on said transistor, said opaque cover also preventing light from reaching said floating gate and removing electrical charge stored on said floating gate,     said transistor further comprising a first insulation layer for insulating said control gate from said opaque cover, a portion of said opaque cover laterally surrounding said transistor and being in direct contact with said semiconductor material,   and wherein said drain extends outwardly beyond the edge of said opaque cover, the portion of said drain at said edge of said opaque cover being separated from said opaque cover by a second insulation layer formed on said drain and a second opaque layer formed between said second insulation layer and said opaque cover, said second opaque layer being formed from the same material as said control gate.   
     
     
       2. The structure of claim 1 wherein said control gate covers the entire area surrounding said floating gate within a distance of 1 micron of said floating gate. 
     
     
       3. The structure of claim 1, wherein said second opaque layer directly contacts said opaque cover. 
     
     
       4. The structure of claim 3 wherein said source extends outwardly beyond said edge of said opaque cover, the portion of said source at said edge of said edge of said opaque cover being separated from said opaque cover by a third insulation layer formed on said source and a third opaque layer formed between said third insulation layer and said opaque cover, said third opaque layer being formed from the same material as said control gate. 
     
     
       5. The structure of claim 1 wherein said second opaque layer extends outwardly beyond said edge of said opaque cover to provide additional shading over said drain. 
     
     
       6. The structure of claim 1 wherein said second insulation layer is less than about 700 Å thick.